In many applications there is a need to keep an operating system within predefined temperature ranges. A common type of temperature sensing device used for this task is known as a “contact device,” which is relatively inexpensive and can be used to accurately measure temperature. A contact device operates in direct contact with a measured object. In this way, the contact device senses the temperature of the object thereby generating a temperature measurement can be obtained.
One type of contact device temperature sensor is based on a semiconductor junction (usually Silicon or Germanium), which can be compactly packaged as a small temperature sensor. The temperature measurement in such devices is based on the strong temperature dependency of the electrical current and voltage of the semiconductor junction. This type of temperature measurement is particularly attractive in integrated circuits (“IC”) in which the sensing device is usually implemented by a bipolar transistor integrated into the semiconductor substrate.
A typical measurement array, based on a semiconductor junction device, is illustrated in FIG. 1. Current source 110 input and the voltage over the base (b) emitter (e) junction of bipolar transistor 100 are utilized for temperature measurement. The forward current If, and forward voltage Vf, of a semiconductor junction, are related by the Ebers-Moll relationship (also known as the Shockley equation):
      I    f    =                    I        s            ·              (                              ⅇ                                          V                f                                            η                ·                                  V                  t                                                              -          1                )              ⁢          →                        V          f                >>                  V          t                      ⁢                  I        s            ·              ⅇ                              V            f                                η            ·                          V              t                                          where Is is the junction saturation current, η is an ideality factor (emission coefficient), and Vt is the junction Thermal Voltage. The Thermal Voltage can be determined according to the junction temperature by the following rule:
            V      t        =                  k        ·        T            q        ,where k is Boltzmann's constant, q is the electron charge, and T is the junction temperature (K°). Since the forward junction current If is constant, the junction temperature may be determined by measuring the forward junction voltage Vf and by computing
      V    t    ⁢            V      f        ·                            (                                    η              ·              ln                        ⁢                                          I                f                                            I                s                                              )                          -          1                    .      The junction temperature T can then be calculated from
  T  =                    q        ·                  V          t                    k        .  
However, due to parasitic effects (e.g., contact resistance), a precise measurement of the junction forward voltage Vf can not be obtained. In fact, the closest measurement of the junction forward voltage can be obtained via the b and e terminals, Vbe, of bipolar transistor 100. The Ohmic resistances rc, rb, and re, illustrated in FIG. 1, are the main obstacle in obtaining an accurate measurement of the forward voltage Vf of the junction.
One method to solve this problem is described in U.S. Pat. No. 5,195,827, where the effect of the Ohmic resistances is eliminated by carrying out a sequence of three voltage measurements corresponding to three different forward currents (If1, If2, and If3). While this method improves the result of the temperature measurement, a significant computation effort (involving log operations) is required for each sequence of measurements to obtain the temperature measurement. Therefore, a central processor is utilized in this device for automatic excitation and temperature calculation. In addition, the computation performed in this method requires that the ratios between the different three forward currents will be substantially large, and a possible embodiment of a current source suggests utilizing three resistors of different resistance values supplied by a common voltage.
The aforementioned drawbacks lead to inaccuracies in the temperature measurement. In addition, using the computation method results in a temperature offset of about one-half (½) of a degree in the calculated temperature.
A temperature measurement application that uses two current sources for measuring the temperature on diode based devices is disclosed in National Semiconductor publication “Design Consideration for PC Thermal Management”. The method described in this publication uses two currents of different magnitudes (x1 and x10) to excite the diode junction. This application however does not provide any way to eliminate Ohmic effects.